Monolayer iron oxide film on platinum promotes low temperature CO oxidation

摘要

CO oxidation on a clean Pt(1 1 1) single crystal and thin iron oxide films grown on Pt(1 1 1) was studied at different CO:O_2 ratios (between 1:5 and 5:1) and partial pressures up to 60 mbar at 400-450 K. Structural characterization of the model catalysts was performed by scanning tunnelling microscopy, low energy electron diffraction, Auger electron spectroscopy and temperature-programmed desorption. It was found that monolayer FeO(1 1 1) films grown on Pt(1 1 1) were much more active than clean Pt(1 1 1) and nm-thick Fe_3O_4(1 1 1) films at all reaction conditions studied. Post-characterization of the catalysts revealed that at CO:O_2 > 1 the FeO(1 1 1) film dewets the Pt surface with time, ultimately resulting in highly dispersed iron oxide particles on Pt(1 1 1). The film dewetting was monitored in situ by polarization-modulated infrared reflection absorption spectroscopy. The reaction rate at 450 K exhibited first order for O_2 and non-monotonously depended on CO pressure. In O_2-rich ambient the films were enriched with oxygen while maintaining the long-range ordering. Based on the structure-reactivity relationships observed for the FeO/Pt films, we propose that the reaction proceeds through the formation of a well-ordered, oxygen-rich FeO_x (1 < x < 2) film that reacts with CO through the redox mechanism. The reaction-induced dewetting in fact deactivates the catalyst. The results may aid in our deeper understanding of reactivity of metal particles encapsulated by thin oxide films as a result of strong metal-support interaction.